SQL> alter table dept modify (deptno number(2) default 10);
how to find out whether a column in a table contains a default value or not.how to see the default values in a table in oracle
Use the metadata table ALL_TAB_COLUMNS (or USER_TAB_COLUMNS if the table in question is on your current schema):
select DATA_DEFAULT
from all_tab_columns
where table_name='xxx' and column_name='yyy'
Related
I have about 50 tables and I would like to know if there is any way to obtain with a query, which columns of my tables are created in a specific tablespace? can you know? or know what things are created in that tablespace?
I guess this might be what you're looking for.
Create a sample table which contains a CLOB datatype column:
SQL> create table test1
2 (id number,
3 text clob
4 );
Table created.
LOB columns can be stored into a different tablespace than the rest of columns; although I didn't specify storage info (so TEXT column resides in the same tablespace as the rest of the columns), querying USER_LOBS returns info you're interested in:
SQL> select column_name,
2 table_name,
3 tablespace_name --> this column
4 from user_lobs
5 where table_name = 'TEST1';
COLUMN_NAM TABLE_NAME TABLESPACE_NAME
---------- ---------- ------------------------------
TEXT TEST1 USERS
SQL>
Another free hint: when you're unsure of where to look for certain things, try to ask the Dictionary. For example:
SQL> select * From dictionary where lower(table_name) like '%lob%';
TABLE_NAME COMMENTS
------------------------------ --------------------------------------------------
ALL_LOBS Description of LOBs contained in tables accessible
to the user
ALL_LOB_PARTITIONS
<snip>
USER_LOBS Description of the user's own LOBs contained in the
user's own tables
<snip>
15 rows selected.
SQL>
Is there is a way to remove a constraint (unique index) based on columns name?
What I would like to do is to remove a constraint where columna name is name, and name_type.
ALTER TABLE MY_TABLE DROP CONSTRAINT NAME_OF_CONSTRAINT;
I don't have a name so I would like to do it this way...
ALTER TABLE MY_TABLE DROP CONSTRAINT **WHERE COLUMN = col1 AND column = col2**
Any syntax to do something like this on a constraint.
I didn't think this was possible with a single statement, but it turns out it is, as shown in the examples in the documentation:
ALTER TABLE MY_TABLE DROP UNIQUE(col1, col2);
A complete example:
ALTER TABLE MY_TABLE ADD UNIQUE (col1, col2);
Table my_table altered.
SELECT CONSTRAINT_NAME, INDEX_NAME
FROM USER_CONSTRAINTS
WHERE TABLE_NAME = 'MY_TABLE';
CONSTRAINT_NAME INDEX_NAME
------------------------------ ------------------------------
SYS_C0092455 SYS_C0092455
ALTER TABLE MY_TABLE DROP UNIQUE(col1, col2);
Table my_table altered.
SELECT CONSTRAINT_NAME, INDEX_NAME
FROM USER_CONSTRAINTS
WHERE TABLE_NAME = 'MY_TABLE';
no rows selected
An alternative approach is to query the USER_CONSTRAINTS and USER_CONS_COLUMNS views to find the matching constraint name - presumably system-generated or you would already know it - and then use that name. If you need to do this as a script then you could query in a PL/SQL block, and plug the found constraint name into a dynamic ALTER TABLE statement.
Do a SELECT * FROM USER_CONS_COLUMNS or ALL_CONS_COLUMNS. This will give you the constraint name for the owner, table and column combination.
Out of the multiple rows returned for a column name, use the constraint names as necessary in the ALTER TABLE ... DROP CONSTRAINT... syntax.
Use dynamic sql to do it for all rows in a loop if you are absolutely sure that you can drop all of them.
This will give you an extra layer of protection so that you don't accidentally drop a constraint that was needed.
Why an entry is created in user_tab_cols when we create a function based on a column of a table?
create table t1(a varchar2(100), b number);
select * from user_tab_cols where table_name = 'T1'; -- Two rows coming
create index idx1 on t1(upper(a));
select * from user_tab_cols where table_name = 'T1'; -- Three rows coming
What is the reason to put an entry in user_tab_cols?
The extra column is a virtual column that Oracle has added to store the value of the indexed expression. From the Oracle documentation:
Oracle Database represents the index expression as a virtual column
You can easily verify in SQL*Plus that the extra column is virtual. In fact, it is also a 'hidden' column:
SQL> select column_name, hidden_column, virtual_column from user_tab_cols where table_name = 'T1';
COLUMN_NAME HID VIR
------------------------------ --- ---
A NO NO
B NO NO
SYS_NC00003$ YES YES
The name of the virtual column may be different on your machine.
user_tab_columns filters out hidden columns, as explained in the Oracle documentation for user_tab_cols. So, if you don't wish to see this column, you can query user_tab_columns instead of user_tab_cols:
SQL> select column_name from user_tab_columns where table_name = 'T1';
COLUMN_NAME
------------------------------
A
B
SQL>
I created the following table
CREATE TABLE PLACE(
POSTCODE VARCHAR(10) PRIMARY KEY,
STREET_NAME VARCHAR(10),
COUNTY VARCHAR(10),
CITY VARCHAR(10));
I want to change the name, county and city from varchar(10) to varchar(20). How do I do that?
ALTER TABLE place
MODIFY( street_name VARCHAR2(20),
county VARCHAR2(20),
city VARCHAR2(20) )
Note that I am also changing the data type from VARCHAR to VARCHAR2 to be more conventional. There is no functional difference at present between the two though the behavior of VARCHAR may change in the future to match the SQL standard.
if you want to change only type of column use below:
ALTER TABLE <table_name> MODIFY (<column_name> <new_Type>)
in your case:
ALTER TABLE place MODIFY (street_name VARCHAR2(20),
county VARCHAR2(20),
city VARCHAR2(20))
If your table has data you could act below:
add a column with new type to table.
copy data from old column to new column.
drop old column.
rename new column to old.
For rename a column use below:
ALTER TABLE <table_name> rename column <column_name> to <new_column_name>
Oracle 10G and later
ALTER TABLE table_name
MODIFY column_name datatype;
A very general example is here to do the same -
Table:
CREATE TABLE TABLE_NAME(
ID NUMBER PRIMARY KEY,
COLUMN_NAME NUMBER NOT NULL, -- Modify with varchar2(20) NOT NULL
.
.
.
);
Step to modify the datatype of COLUMN_NAME from NUMBER to VARCHAR2
STEPS:
--Step 1: Add a temp column COLUMN_NAME_TEMP in table TABLE_NAME to hold data temporary
ALTER TABLE TABLE_NAME
ADD( COLUMN_NAME_TEMP varchar2(20) );
--Step 2: Update temp column COLUMN_NAME_TEMP with Old columns COLUMN_NAME data
UPDATE TABLE_NAME
SET COLUMN_NAME_TEMP = COLUMN_NAME;
--Step 3: Remove NOT NULL constrain from old columns COLUMN_NAME
ALTER TABLE TABLE_NAME MODIFY (COLUMN_NAME NULL);
--Step 4: Update old columns COLUMN_NAME data with NULL
UPDATE TABLE_NAME SET COLUMN_NAME = NULL;
--Step 5: Alter table old columns COLUMN_NAME to new data type varchar2(20)
ALTER TABLE TABLE_NAME MODIFY COLUMN_NAME varchar2(20);
--Step 6: Update old columns COLUMN_NAME with data from temp columns COLUMN_NAME_TEMP
UPDATE TABLE_NAME
SET COLUMN_NAME = COLUMN_NAME_TEMP;
--Step 7: Add NOT NULL constrain from old columns [COLUMN_NAME]
ALTER TABLE TABLE_NAME MODIFY (COLUMN_NAME NOT NULL);
--Step 8: Drop the temp column [COLUMN_NAME_TEMP]
alter table TABLE_NAME drop column COLUMN_NAME_TEMP;
If NOT NULL constrain is not exist the omitte step-3 and step-7
Alter table placemodify(street name varchar2(20),city varchar2(20)
You can't modify the data type of a table if you have some amount of records already present in the table.
You have to empty the table records of the column (you want to modify the data type) first and then use the below command :
alter table place
modify ( street_name varchar2(20), country varchar2(20), city varchar2(20) );
Definitely it will work!
I just came across NULL values in NOT-NULL fields in our test database. How could they get there? I know that NOT-NULL constraints can be altered with NOVALIDATE clause, but that would change table's last_ddl_time in USER_OBJECTS. And that time is less than the date that those records were created. Is there something else I'm overlooking? Or is that someone's manual work for sure?
Table is partitioned and index-organized if that is relevant.
Oracle version is 9.2
The NOT NULL column condition is not like other constraints: you can disable a NOT NULL constraint but the column won't be considered NOT NULL if you reenable the constraint with NOVALIDATE. Let's build a small example:
SQL> CREATE TABLE tt (ID NUMBER NOT NULL);
Table created
SQL> SELECT column_name, nullable FROM user_tab_columns WHERE table_name = 'TT';
COLUMN_NAME NULLABLE
------------------------------ --------
ID N
now if I disable the constraint and reenable it with NOVALIDATE, the column won't be considered NOT NULLABLE by Oracle:
SQL> SELECT constraint_name, search_condition
2 FROM user_constraints WHERE table_name = 'TT';
CONSTRAINT_NAME SEARCH_CONDITION
------------------------------ ----------------------------
SYS_C00786538 "ID" IS NOT NULL
SQL> ALTER TABLE tt MODIFY CONSTRAINT SYS_C00786538 DISABLE;
Table altered
SQL> ALTER TABLE tt MODIFY CONSTRAINT SYS_C00786538 ENABLE NOVALIDATE;
Table altered
SQL> SELECT column_name, nullable FROM user_tab_columns WHERE table_name = 'TT';
COLUMN_NAME NULLABLE
------------------------------ --------
ID Y
So, I would say that if you have NULL values in a NOT NULLABLE column (as per my last query) you have a bug (contact support?)
Check If the constraint are suspended / Disabled
And you're sure these columns are really null? In other words:
SELECT field
FROM your_table
WHERE not_null_field IS NULL;
returns rows? Perhaps they've just got non-displayable data...